CNA Explains: Do you need an air purifier for your home if haze hits?

Singapore

An air purifier should be able to filter out small particles with efficiency, have a high clean air delivery rate and have carbon filters to deal with the smell of haze.

(Photo: iStock/Jomkwan)

SINGAPORE: With a high risk of severe transboundary haze this year, consumers are turning to air purifiers to prepare for bad weather conditions.

Used indoors, air purifiers – or air cleaners – remove contaminants from the air you breathe in.

While they were commonly used during past haze episodes in Singapore, those thinking of reusing old devices should check if they are working well.

CNA spoke to five experts to find out if air purifiers are effective, what you should prioritise when purchasing one and how to test an existing one for its effectiveness.

Even if you keep all windows in your home closed, tiny particles from haze can still enter gaps and infiltrate your indoor environment, said the National University of Singapore's (NUS) Professor Rajasekhar Balasubramanian.

Also, keeping windows and doors shut cuts off natural ventilation and allows particles to accumulate, the professor from the Department of Civil and Environmental Engineering said.

An air purifier will remove particles through filters, improving the quality of air.

The key air pollutant of concern during haze is PM2.5, or fine particulate matter, with a diameter of 2.5 micrometres or smaller. Each particle is about 300 to 400 times smaller than human hair, said Prof Balasubramanian.

To deal with contaminants of such a small size, buyers should look for high-efficiency particulate air (HEPA) filters, or those with high minimum efficiency reporting values (MERV) rating, experts said.

Prof Balasubramanian said to take note of the particle size the air purifier is designed to remove.

"We are talking about less than 2.5 microns or micrometres, or sometimes it could be even smaller than 1 micron. So it is important to ensure that the air purifier that you are considering purchasing has been designed to remove such small particles efficiently."

Three experts from the Indoor Air Quality Research Unit at NUS' Department of the Built Environment said HEPA targets 0.3-micron particles, which is the most penetrating particle size.

"By definition a HEPA filter removes 99.97 per cent of 0.3 microns size. Transboundary haze is essentially very small particles, with a large proportion near the 0.3 microns size range," Associate Professor Tham Kwok Wai, Associate Professor David Cheong and Professor Chandra Sekhar said.

Nanyang Technological University (NTU) Associate Professor Steve Yim said: "People with respiratory conditions may need filters with a larger MERV rating. For MERV rating of at least 14, the filter can remove at least 75 per cent of particulates with a size between 0.3 to 1 micron and 90 per cent with size 1 to 3 microns."

Another factor to look out for in an air purifier: The clean air delivery rate (CADR), which measures the purifier's effectiveness based on room space and the volume of clean air produced.

This shows how well dust, pollen and smoke – the three most common indoor air pollutants – are removed from the air you breathe, the three NUS experts said.

National Environment Agency (NEA) guidelines state that the smoke CADR of an air purifier, measured in cubic metres per hour, should be at least five times the volume of the room, measured in cubic metres.

While you might be tempted to buy a large air purifier to cover a bigger space, such an air purifier not only costs more, but can be noisier and consume more power. Instead, consider using more than one small air purifier, located at separate ends of a large room, Prof Balasubramanian said.

Experts also recommended buying a device with a carbon filter to deal with the strong smell of haze.

"Carbon filters remove the smell associated with several combustion byproducts that renders haze with a characteristic smell. Activated carbon absorbs these," said the three NUS experts.

Prof Balasubramanian said the strong smell of haze is associated with organic compounds in the gas phase, and these are not efficiently removed by HEPA filters. A combination of both HEPA and carbon filters will protect the user better, he added.

Some air purifiers generate additional air pollutants, like ozone, which are health hazards.

According to NEA's website, electronic portable air cleaners that use ionizers, photocatalytic oxidizers, electrostatic precipitators, hydroxyl generators, UV light or other electronic air-cleaning technologies may generate harmful levels of ozone or ions.

Mechanical filters such as HEPA do not generate ozone or ions.

It may have been years since you last used your air purifier. Can you still use it if haze hits this year?

Assoc Prof Yim, who is from NTU's Asian School of the Environment, said it depends on how an air purifier has been maintained. "Old filters should be regularly changed to a new one for a certain time period depending on the indoor air quality."

Assoc Prof Tham, Assoc Prof Cheong and Prof Sekhar said there was little a layperson could do to evaluate their air purifiers' performance, as the process is highly technical and done in the laboratory. However, users can still replace the filters.

"Activated carbon must be viewed as something that depletes over time. Its capacity to absorb chemicals reduces when in continual use. When one starts to detect smell penetrating the filter, this is indicative that the activated carbon has lost its ability to remove such chemicals," they said.

Prof Balasubramanian cautioned users against changing filters in the presence of others, especially those who suffer from respiratory conditions, and to also wear an N95 mask while doing so.

Asked if there was a way to check for an air purifier's effectiveness, the NUS professor said it was "very hard" but suggested using a cheap particle counter to measure the amount of particles in an enclosed room before and after using the air purifier.

Alternatively, he suggested to try lighting an incense stick. If an air purifier is working well, the smell and amount of smoke emitted from the incense stick should be less than if the air purifier is not switched on.

It should, but beware of devices that generate ozone, said Prof Balasubramanian.

Some air purifiers used for COVID-19 disinfect by inactivating the virus through germicidal lamps, or UV lamps, that generate ozone.

NUS' Assoc Prof Tham, Assoc Prof Cheong and Prof Sekhar said: "Unless the CADR is compromised to achieve a better efficacy against COVID-19 virus, it should continue to be effective against the haze.

"Again, it is good to remember that haze is a complex contaminant with physical, chemical and possibly biological components. An air cleaner should adequately address all these components to be effective against the haze."